At present, LEDs (light-emitting diodes) of various types, such as those with high luminance of light emission, and those emitting white light and various colors of light have been developed and are in mass production, and have found wide application in various fields, such as backlighting, illumination, displays, etc.
FIG. 11 is a diagram illustrating the circuit constitution of a conventional LCD device in the prior art for use as a backlight in LCD (liquid crystal display)-TV (television) applications. As shown in the figure, this LCD device has LED array 12 consisting of n×m LEDs (10(0,0), . . . 10(n-2,0), 10(N-1,0))-(10(0,m-1), . . . 10(n-2,m-1), 10(N-1,m-1)) (where n and m are integers of 2 or more) and one or a plurality (N) of LED driver ICs (integrated circuits) 14(0)-14(N−1) of, e.g., the 16-channel type, a DC power source, such as DC-DC converter 16, and controller 18.
As shown in FIG. 11, in each column, LEDs 10(0,y), . . . 10(n-2,y), 10(N-1,y) (y=0 to m−1) are electrically connected in series between the output terminal of DC-DC converter 16 and the corresponding current terminals OUTy of LED driver IC 14. For example, as the first column, LED 10(0,0), . . . 10(n-2,0), 10(N-1,0) are electrically connected in series between the output terminal of DC-DC converter 16 and first current terminal OUT0 of first LED driver IC 14(0). On the other hand, as the mth column, LED 10(0,m-1), . . . 10(n-2,m-1), 10(N-1,m-1) are electrically connected in series between the output terminal of DC-DC converter 16 and the tail current terminal OUTm-1 used in the Nth LED driver IC 14(N−1).
For said LED backlight, an area-light system is adopted, and, as shown in FIG. 12, backlight region 22 is divided in matrix configuration into m (m=i×j) blocks B0, B1, . . . Bm-1, and, in each block By, the various corresponding column LEDs 10(0,y), . . . 10(n-2,y), 10(N-1,y) shown in FIG. 11 are set two-dimensionally with a constant density distribution as shown in FIG. 13.
In FIG. 11, DC-DC converter 16 is a switching power source that works as, e.g., a chop method voltage boosting type converter. For example, it boosts DC input voltage VIN input as 24 V voltage to a DC voltage at a prescribed level of, e.g., 50 V, that is output as LED driving voltage VLED.
Said DC-DC converter 16 performs constant-voltage control for its output voltage, that is, LED driving voltage VLED. For this purpose, it has reference voltage input terminal REF, feedback voltage input terminal FB, and a feedback circuit consisting of voltage dividing resistors 24, 26. More specifically, said voltage dividing resistors 24, 26 are connected in series between the output terminal of DC-DC converter 16 and the ground potential terminal. Node NA between the two resistors is connected to feedback voltage input terminal FB. Assuming that the resistances of said voltage dividing resistors 24, 26 are R24 and R26, voltage divided voltage VA obtained by multiplying coefficient R26/(R24+R26) with LED driving voltage VLED is obtained at node NA. Said voltage divided voltage VA is input as feedback voltage to feedback voltage input terminal FB. On the other hand, a prescribed reference voltage VREF is input from controller 18 to reference voltage input terminal REF. Said DC-DC converter 16 performs the operation of a switching power source so that feedback voltage VA from voltage dividing circuit (24, 26) is equal to reference voltage VREF.
Each of LED driver ICs 14(x) (x=0 to N−1) has a 16-channel sink type constant current driving circuit. The output terminals of the various constant current driving circuits are taken as said current terminals OUTy (y=0 to m−1). The constant current driving circuit of each channel works so that a prescribed LED driving current Iy flows in LEDs 10(0,y), . . . 10(n-2,y), 10(N-1,y) of the corresponding column. Here, in order to guarantee stable constant current operation, a voltage over the prescribed level should be kept as headroom voltage HVy at each of current terminals OUTy, and the output voltage of DC-DC converter 16, that is, LED driving voltage VLED is set so that said headroom voltage condition is met. Here, said headroom voltage HVy at each current terminal OUTy is represented by HVy=VLED−Vy(0 to N-1), where Vy(0 to N-1) represents the total voltage fall generated in the corresponding LED serial circuit (10(0,y), . . . 10(n-2,y), 10(N-1,y)).
Together with a desired clock signal from controller 18, the data and control signal for controlling the brightness of the LED backlight are input to each LED driver IC 14(x). For a recently developed LCD-TV unit, the local dimming scheme is adopted. According to this scheme, for the image on each frame, the brightness of the LED backlight is under variable control in units of area or blocks. In order to perform said local dimming, grey scale data indicating the luminance or brightness degree of each block By are sent in serial transfer to the constant current driving circuit from controller 18 at a constant cycle (e.g. 120 Hz), and each constant current driving circuit works based on each grey scale datum to variably control the ON time of LED driving current Iy in each cycle, that is, the duty, with a PWM (pulse width modulation) control system.
As shown in FIG. 11, NMOS transistor 28 is set for protecting each constant current driving circuit from high voltage in case of an LED short circuit since it is connected between LEDs 10(0,y), . . . 10(n-2,y), 10(N-1,y) and the corresponding current terminals OUTy. Said NMOS transistor 28 is biased to bias voltage Vk provided by the voltage dividing circuit consisting of resistors 30, 32, and the voltage of each current terminal OUTy is restricted to a prescribed level of (Vk+Vth) or lower. Here, Vth represents the threshold voltage of NMOS transistor 28.
Usually, the forward voltage of an LED has negative temperature characteristics. The lower the temperature of the LED, the larger the voltage decrease generated in the LED in the light emission state, and the lower the headroom voltage HVy obtained at each current terminal OUTy in LED driver IC 14(x). Consequently, output voltage VLED of DC-DC converter 16 is set so that headroom voltage HVy over a prescribed level is guaranteed at each current terminal OUTy even at a temperature lower than the lowest operating temperature of the LCD-TV.
On the other hand, when the temperature of an LED rises due to a rise in the ambient temperature or due to self-heating of the LED, the voltage decrease at the LED in the light emission state decreases, and, corresponding to this, headroom voltage HVy at each current terminal OUTy in LED driver IC 14(x) rises. This is undesired. That is, each constant current driving circuit works such that a prescribed LED driving current Iy flows. Consequently, the higher the headroom voltage HVy, the higher the power consumption of the constant current driving circuit. In addition, when the overall power consumption (heat generation quantity) of LED driver IC 14(x) is over the permissible loss of the IC package, the driver circuit is broken or malfunctions so that normal operation cannot be performed, and the reliability falls.